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Sautrântika theory of Inference [Footnote ref 1].
According to the Sautrântika doctrine of Buddhism as described by Dharmakîrtti and Dharmmottara which is probably the only account of systematic Buddhist logic that is now available to us in Sanskrit, inference (anumâna) is divided into two classes, called svârthânumâna (inferential knowledge attained by a person arguing in his own mind or judgments), and parârthânumâna (inference through the help of articulated propositions for convincing others in a debate). The validity of inference depended, like the validity of perception, on copying the actually existing facts of the external world. Inference copied external realities as much as perception did; just as the validity of the immediate perception of blue depends upon its similarity to the external blue thing perceived, so the validity of the inference of a blue thing also, so far as it is knowledge, depends upon its resemblance to the external fact thus inferred (sârûpyavas'âddhi tannîlapratîtirûpam sidhyati).
The reason by which an inference is made should be such that it may be present only in those cases where the thing to be inferred exists, and absent in every case where it does not exist. It is only when the reason is tested by both these joint conditions that an unfailing connection (pratibandha) between the reason and the thing to be inferred can be established. It is not enough that the reason should be present in all cases where the thing to be inferred exists and absent where it does not exist, but it is necessary that it should be present only in the above case. This law (niyama) is essential for establishing the unfailing condition necessary for inference [Footnote ref 2]. This unfailing natural connection (svabhâvapratibandha) is found in two types
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[Footnote 1: As the Pramâ@nasamuccaya of Diñnâga is not available in Sanskrit, we can hardly know anything of developed Buddhist logic except what can be got from the Nyâyabindu@tîkâ of Dharmmottara.]
[Footnote 2: tasmât niyamavatorevânvayavyatirekayo@h prayoga@h karttavya@h yena pratibandho gamyeta sâdhanyasa sâdhyena. Nyâyabindu@tîkâ, p. 24.]
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of cases. The first is that where the nature of the reason is contained in the thing to be inferred as a part of its nature, i.e. where the reason stands for a species of which the thing to be inferred is a genus; thus a stupid person living in a place full of tall pines may come to think that pines are called trees because they are tall and it may be useful to point out to him that even a small pine plant is a tree because it is pine; the quality of pineness forms a part of the essence of treeness, for the former being a species is contained in the latter as a genus; the nature of the species being identical with the nature of the genus, one could infer the latter from the former but not vice versa; this is called the unfailing natural connection of identity of nature (tâdâtmya). The second is that where the cause is inferred from the effect which stands as the reason of the former. Thus from the smoke the fire which has produced it may be inferred. The ground of these inferences is that reason is naturally indissolubly connected with the thing to be inferred, and unless this is the case, no inference is warrantable.
This natural indissoluble connection (svabhâvapratibandha), be it of the nature of identity of essence of the species in the genus or inseparable connection of the effect with the cause, is the ground of all inference [Footnote ref 1]. The svabhâvapratibandha determines the inseparability of connection (avinâbhâvaniyama) and the inference is made not through a series of premisses, but directly by the li@nga (reason) which has the inseparable connection [Footnote ref 2].